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PostPosted: Mon Nov 18, 2013 5:32 pm 
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Joined: Wed Dec 05, 2012 6:52 pm
Posts: 79
Here is the figures you need and the method I use.
The HW40 and P17 require a build up of 1.25mm before piston bind becomes a problem. If you get piston bind, it can destroy the guns performance and ruin the gun so be careful.
I will explain....
At 1.40mm the piston bottoms out at the end of the chamber before the top strap seats down. You will still manage to close the top strap but flex tension is introduced into the components. This means that the pivot pin is forced a few microns forward, which in turn moves the barrel breech interface forward, reducing the seal at the breech.
Given slight differences from gun to gun, some might still manage to close without issue but some maybe put under even greater tension causing a complete breech seal failure. Thus I worked on adding 1.25mm for a safety margin.

My method started in reverse by machining a spot face 1mm deep x 15mm dia into the head of the piston. I then machined up an Aluminium disk of 2.5mm thick x 15mm dia (about like an English penny coin, or a Dime) and glued it into place. It already had a slight drive fit as it turned out but i added the glue to be sure. I then set the piston in the lathe and machined 0.25mm off the front.
I have performed this mod to both models for a full 30fps (26 fps low...31fps high) from both guns for a muzzle velocity of 422 fps with Hobby pellets HW40 and 425 fps P17 (high)

My other mod did not come from the valve head mod discussed here (I have not performed this mod) but from adding a longer barrel. My barrel does not protrude past the muzzle as I feel such mods spoil the looks of the gun but I ran it through the compensator void to the true muzzle flush with the end. I got 12fps low and 15fps high (HW40 only....I haven't done the barrel mod for the P17. I now have a HW40 nudging the 440fps area.

I have achieved the greatest increase from similar work with a HW75 employing a good extra length of barrel through the longer recess in this model. Still not protruding past the muzzle but twice the length I could manage through the shorter recess of the 40.
The piston mod was performed similarly but in steel of course.
460fps with the Hobby (high)


Last edited by clarky on Mon Nov 18, 2013 7:12 pm, edited 1 time in total.

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PostPosted: Mon Nov 18, 2013 6:13 pm 
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Thanks for chiming in with theses number Clark very nice of you, the thickness of a dime sounds about right. :wink:

The thing is not everyone has access to such machinery to make such thin peaces of aluminum and then gluing it on. The aluminum foil tap will give you the same performance with a lot less work and is fully reversible if desired. :wink:

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PostPosted: Mon Nov 18, 2013 7:08 pm 
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Yes I appreciate its a good method if you do not have a lathe, while these modern metal glue systems can be perfectly strong enough. Besides there is no tension on the mod, except compressive forces acting in the right direction.
I just wanted to perform a pucker engineer job on mine.


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PostPosted: Mon Nov 18, 2013 7:35 pm 
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clarky wrote:
Yes I appreciate its a good method if you do not have a lathe, while these modern metal glue systems can be perfectly strong enough. Besides there is no tension on the mod, except compressive forces acting in the right direction.
I just wanted to perform a pucker engineer job on mine.


exactly thats why i say the tape is the easiest and least expensive way to go. :D

Clark have you see this :wink:
http://www.network54.com/Forum/275684/m ... roke+PICS-

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PostPosted: Tue Nov 19, 2013 2:08 pm 
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We agree on that. I may look at trying your valve mod which looks very interesting, particularly to see if this will work for the HW75 that I have already modified.
That mod over on that forum you linked received so much criticism for the simple reason that it will not work!
The components itemised with photographs show people with reasonable multi pump pneumatic knowledge that something is amiss here.....putting it politely.
Thus the individual is either scamming for money or testing the water for the potential of such a mod but regardless it will not function. Sorry about that my friend.


Last edited by clarky on Tue Nov 19, 2013 2:58 pm, edited 1 time in total.

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PostPosted: Tue Nov 19, 2013 2:28 pm 
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clarky wrote:
We agree on that. I may look at trying your valve mod which looks very interesting, particularly to see if this will work for the HW75 that I have already modified.
That mod over on that forum you linked received so my criticism for the simple reason that it will not work!
The components itemised with photographs show people with reasonable multi pump pneumatic knowledge that something is amiss here.....putting it politely.
Thus the individual is either scamming for money or testing the water for the potential of such a mod but regardless it will not function. Sorry about that my friend.


I'm intrigued. It looks like it would work to me - what's the flaw?


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PostPosted: Tue Nov 19, 2013 3:14 pm 
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Because the individual has demonstrated components for a very simply one way valve which may well work for passing multi pumps of air through it but it completely omits the requirements for the equal and opposite handling of the higher pressure air being forced into the existing (unmodified) reservoir, together with the increased hammer requirements for the release valve mechanics. already close to their limits of function.
As example, during my increase of compression at max potential of 1.4mm piston head build up (and close to straining of the mechanics) I noticed an occasional lock down of the system where the guns air release mechanism seized....suggesting I was already getting toward the edge of things. Now factor in twice the pressure build up!
The existing valve and release mechanics are of a design typical of SSP operating considerably below the pressure of say the tiny crosman reservoir, with high pressure poppet valving.
Thus this individual has demonstrated an idea for a multi pump valve....a concept of how something of this type might fit but without consideration to handling the 600 PSI pressures imparted.
It wont function.


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PostPosted: Tue Nov 19, 2013 4:21 pm 
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OK, that sounds reasonable. I'd say it's more a case of "might not function at some point" rather than won't function, depending on the pressure achieved for a given number of pumps. Since the swept volume for an individual stroke is less than the original design, and the lost volume looks likely to be larger, it's pretty certain that the 1 stroke pressure and achieved fps will be lower with this mod than in the original. So it becomes a question of when does it come back even - two pumps, three? And when would the pressure become high enough to possibly cause lock up. With the p17's full dump design it won't lock up in quite the same way that a PCP or CO2 valve will where you have to overcome the pressure on the valve head to release gas. But your experience shows that it will lock up at some point. Interesting!


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PostPosted: Tue Nov 19, 2013 10:55 pm 
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well i cant say that it worked or not but it seams plausible.
It looks like he took this idea and tried to do the same to the P-17
http://www.network54.com/Forum/405945/t ... t%3B.......

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PostPosted: Sun Nov 24, 2013 12:49 pm 
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Joined: Mon Mar 09, 2009 6:46 pm
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Location: r Alberta
Well I tried it with the JB weld and it is still working after about 100 shots. I only picked up about 30 fps. This increase has made a difference to the guns performance and find this idea is worth the time it takes .Thanks for the great ideas.
I find this a great little pistol for indoors and hard to beat for $50.


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PostPosted: Mon Nov 25, 2013 5:06 pm 
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Glad you managed to get the metal glue to work. 30 fps is a decent increase. I only managed similar with the full machining job I did but both good mods getting the gun into Webley Tempest velocities but recoilless of course.....a far better pistol for being so IMO, in either variant.
Although a tad tougher to cock I believe my accuracy has increased marginally. Could be the flatter trajectory or the faster lock time but I've repeatedly measured 1mm smaller groups for weeks since doing mine. Its only slight but great news because the slight extra power has not spoilt accuracy but slightly improved it.
The HW40 completely trounces the Tempest and HW45 in accuracy.


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PostPosted: Mon Nov 25, 2013 6:24 pm 
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Joined: Mon Mar 09, 2009 6:46 pm
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Location: r Alberta
I found that it extended the rang I could shoot by about 5 yards and still maintain the same accuracy. We weren't paper punching just shooting knock down ducks in a resetable target.


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PostPosted: Sat Dec 14, 2013 2:51 pm 
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Joined: Fri Sep 21, 2012 10:23 am
Posts: 78
Location: Sackville Nova Scotia
When I measured my p17 piston depth I had less then a mm space between the end of the chamber and the piston face. I used a digital caliper to measure this and it was soooo small that I think 2 layers of tape would have made it contact. Having said that my p17 seemed to have a fair bit of kick. Id say it wasn't far off from 8 pumps on a 1377 stock.

If I were to do another one id double o ring the piston and make it slightly adjustable for length and add a longer barrel.


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